Graphite-glass composite laser mirror

ABSTRACT

A laser mirror stable at high and low temperatures is described comprising graphite fibers in a glass matrix. The mirror is made by hot pressing a graphite fiber lay-up in a glass matrix in a die, the fibers laid up in such a way to produce a central plane of symmetry across the central plane of the composite. In use, the composite requires a separate laser reflecting surface layer. The resulting laser mirror has a low density but no porosity, high elastic stiffness, high strength, high fracture toughness, low thermal expansion, high thermal conductivity and environmental stability.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The field of art to which this invention pertains is composite opticalelements of the reflecting type.

2. Description of the Prior Art

While there is a myriad of art covering laser mirrors (e.g., U.S. Pat.Nos. 3,836,236; 3,926,510; and 3,942,880) because of the many peculiarphysical property requirements of such mirrors in this environment, botha variety of materials and designs have been employed in attempts tooptimize the particular properties necessary for a composite used inthis particular environment. For example, a laser mirror in thisenvironment must not only have the requisite reflective properties butshould be a relatively simple structure to permit rapid fabrication,both for time and cost purposes. Such mirrors should also desirably havelow density for ease of use in the types of apparatus where they will beused. Furthermore, such mirrors ideally should have high elasticstiffness and high strength along with high fracture toughness. Andstability is of the utmost importance both from the point of view of thefine resolution-type work environment the mirrors will be used in, andthe inaccessibility of the apparatus which these mirrors would be usedin, for example outer space applications. These stability propertiesinclude low thermal expansion, high thermal conductivity andenvironmental stability. Environmental stability includes such things asdimensional stability and mirror integrity regardless of moistureconditions, vacuum conditions, or ultraviolet light exposure, and mirrorintegrity and dimensional stability at both high and low temperatures.Currently, laser mirrors are basically either highly polished metalblocks (high energy laser application), or graphite reinforced resinmatrix composites or low expansion glasses (low energy laserapplication). However, because such currently used composites andglasses fall off in one or more of the above-cited property areas, it isgenerally necessary to include a myriad of complicated designs tocompensate for property defects in the particular areas underconsideration. Note, for example, the complicated coolant designs in theabove-cited references. Furthermore, the popular use of resins inconventional composites of the above type inherently suffer fromdimensional changes due to absorption or desorption of moisture,evolution of organic constituents due to prolonged exposure to highvacuum, breakdown due to prolonged exposure to ultraviolet radiation,low thermal conductivity, high coefficients of thermal expansion, andrapid decrease in integrity when used above 300° C.

BRIEF SUMMARY OF THE INVENTION

The present invention is directed to laser mirrors comprising graphitefibers in a glass matrix having a central plane of symmetry across theircentral plane. Both the graphite fibers and the glass matrix areselected and matched to provide, after hot pressing in a mold, a lasermirror of low density, high elastic stiffness, high strength, highfracture toughness, low thermal expansion, high thermal conductivity andenvironmental stability. The graphite fibers must have high strength andgood modulus of elasticity. The glass matrix must have a low coefficientof thermal expansion matched closely to that of the graphite fiber. Themirror is made by laying alternate layers of fiber and glass and hotpressing at elevated temperatures to form a composite mirror having acentral plane of symmetry across its central plane. The composite itselfmay provide the reflecting surface shape. However, in use the compositerequires a laser reflecting surface layer. The thermal expansioncharacteristics, thermal conductivity and composite strength andfracture toughness of such composite resulting from the selection ofsuch materials processed as described is far superior to any similararticles currently in use, which is quite surprising from the relativelybrittle nature of the two starting materials.

The foregoing and other objects, features and advantages of the presentinvention will become more apparent from the following description ofpreferred embodiments and accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 demonstrates a typical 0°/90° cross-ply graphite fiber lay-up forthe mirrors of the present invention.

FIG. 2 demonstrates an end view of a finished mirror of the presentinvention.

FIG. 3 demonstrates graphically thermal expansion characteristics ofmirrors of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

While any graphite fiber with the requisite high strength and goodmodulus of elasticity can be used in the laser mirrors of thisinvention, such as Hercules HMS graphite fiber, Celanese GY-70 (formerlyDG102) graphite fibers are particularly suitable. This fiber consists of384 fibers/tow and has an oxidized finish. It is 8 microns in diameter,has a modulus of elasticity of 531 GPa (77(10)⁶ psi). It has a tensilestrength of 1724 MPa (250 ksi) and a density of 1.96 gm/cm³. The fiberis used at about 40 to 70% by volume based on the graphite-glasscomposite and preferably at about 60% by volume.

The glass used was particularly selected to have a very low coefficientof thermal expansion preferably matched closely, but not equal to thatof the graphite fibers used since the graphite has a highly negativeaxial coefficient of thermal expansion and the glass has a positive butsmall coefficient of thermal expansion. Particularly suitable for thepurposes of this invention is a borosilicate glass (CGW 7740) with ananneal point of 500° C., a softening point of 821° C., a liquidustemperature of 1017° C., a density of 2.23 grams per cubic centimeter,an index of refraction of 1.474, a dielectric constant of 4.6, acoefficient of linear expansion of 32.5 cm/cm°C.×10⁻⁷ and a modulus ofelasticity of 9.1×10⁶ psi. The particle size of the glass should be suchthat at least 90% passes through a 36.0 mesh screen.

While a variety of methods may be used to produce the laser mirror ofthe present invention, the preferred method comprises continuouslyunwinding a tow of graphite fibers from a spool at a moderate rate ofspeed and passing such fibers through a slip of powdered glass, solventand plasticizer to impregnate the tow. The impregnated fibers are thenrewound onto a larger rotating spool. An exemplary slip composition maybe composed of 250 grams of powdered glass in 780 ml of propanol. Analternative composition may comprise 85 grams of the glass and 200 gramsof propanol, 10 grams of polyvinyl alcohol and 5 drops (approx. 1 cc) ofa wetting agent, Tergitol®. The receiving drum is preferably run at 1revolution per minute or linear speed of 5 feet per minute. Excess glassand solvent can be removed by pressing a squeegy against the drum as itwinds. Preferably the ground glass is sized so that 90% of it passesthrough a 325 mesh sieve. The thus impregnated tape is then dried eitherat ambient temperature or with a radiant heating source to removesolvent.

Following the impregnation the fiber is removed from the drum and cutinto strips up to the diameter of the mirror to be fabricated. While thetypical test samples made were about 10 cm in diameter, mirrors up to 20cm in diameter have also been made by the processes of the presentinvention. However, mirrors of even larger diameters can be madeaccording to the present invention. The fibers are then preferably laidin alternating ply stack-up sequence of 0 and 90° as illustrated byFIGS. 1 and 2. The assembled composite is then hot pressed, either undervacuum or inert gas such as argon, in metal dies coated with colloidalboron nitride, or graphite dies sprayed with boron nitride powder, atpressures of 6.9 to 13.8 MPa (1000-2000 psi) and temperatures of1050°-1450° C. Additional glass in the form of powder may also beinserted between each layer as it is laid in an attempt to achieve apreferred 40-70% by volume loading of graphite fiber in the composite.Also, the mold can be vibrated to insure uniform distribution of theglass over the laid fiber surfaces.

It is important for obtaining the above-cited mirror properties that acentral plane of symmetry be maintained across the central plane of thegraphite glass composite. Thus, while alternating 0 and 90° fiber layingwas the most common test sample arrangement used, 0 and 45°; 0, 45 and90°; 0, 30 and 90°; 0 and 60°, etc., fiber laying can also be used aslong as a central plane of symmetry is maintained. In fact, the 0, 45and 90° and 0 and 60° fiber laying give additional advantage of isotropyof elastic stiffness. By central plane of symmetry is meant that if thecomposite is sliced in half parallel to what will be the reflectingsurface and perpendicular to the mirror's focal axis, each half will bea mirror image of the other half. Such symmetry especially affects thewarpage characteristics of the composite. In addition, it is preferredthat the ply lay-up sequence be balanced. By balanced it is meant thatthere are equal numbers of composite plies, and hence fibers, directedin each of the principal fiber directions chosen. Such balance affectsthe strength and modulus properties of the composite. This isspecifically demonstrated by FIG. 2 which is the end view of a 0°/90°specimen where 1 indicates the glass matrix, 2 indicates the graphitefibers and 3 indicates the central plane of symmetry.

As can be seen from FIG. 3, the thermal expansion characteristics of a0° and 90° graphite-glass lay up are on a par with other conventionalmaterials known for their dimensional stability. Curve A representsunreinforced fused silica (N.B.S. reference number 739 measurement).Reference B represents unreinforced ULE glass (Corning Code 7971measurement). And Reference C represents the 0° and 90° graphite glassof the present invention. Because of its superiority in other propertiesof strength, stiffness, thermal conductivity, and toughness, thegraphite reinforced glass is the overall superior material.

The finished mirror composite can be so molded as to provide thereflecting surface shape itself, e.g., by using a highly polishedreleasable mold surface. However, in use it is necessary to provide themirror composite with a separate laser reflecting surface such as areconventionally used in the laser mirror art (e.g., a chromium-goldalloy). Such surface layers can be applied by conventional methodscommonly used to deposit thin uniform layers (e.g., less than 1 mil) ofsuch material such as spraying, vapor deposition and cathode sputtering.

As stated above, in the Summary of the Invention, process parameters andmaterials are specifically selected in the laser mirror application toprovide ease of rapid fabrication, low density, high elastic stiffness,high strength, high fracture toughness, low thermal expansion, highthermal conductivity and environmental stability. Most important ofthese characteristics are the thermal expansion, thermal conductivity,and composite strength and fracture toughness. Note also Table I for acomparison of coefficients of thermal expansion for composites ofvarious matrices.

                  TABLE I                                                         ______________________________________                                                                             Tem-                                                        0° CTE                                                                           90° CTE                                                                        perature                                 Fiber    Matrix    (10.sup.-6 /°C.)                                                                 (10.sup.-6 /°C.)                                                               (°C.)                             ______________________________________                                        50% Hercules                                                                           7740 Pyrex                                                                              -0.51      4.5    27                                       HMS      Glass                                                                50% Hercules                                                                           7740 Pyrex                                                                              -0.62      4.5    -125                                     HMS      Glass                                                                50% Hercules                                                                           Epoxy     -0.18     37.4    24 to +180                               HMS                                                                           50% Hercules                                                                           Epoxy     -0.56     24.3    24 to -200                               HMS                                                                           Hercules Polyimide -0.306    --      22                                       HMS                                                                           ______________________________________                                    

The thermal conductivity of a laser mirror composite formed according tothe present invention is also high. For example, the thermalconductivity through a plane of a two-dimensional composite is clearlybetter than conventional epoxies and nearly twice that for the samecomposite using a conventional polyimide matrix rather than a glassmatrix. Attention is directed to Table II for comparison of the superiorthermal conductivity of the glass composite of the present invention ascompared to that of resin matrix composites.

                  TABLE II                                                        ______________________________________                                                                   Thermal                                                                       Conductivity                                       Material      Orientation  (cal/sec-cm-°C.)                            ______________________________________                                        HMS Fiber (0/90)-                                                              7740 Glass   ⊥       0.0053                                                           //           0.0700                                             HMS Fiber (0/90)-                                                                           |                                                       Polyimide Resin                                                                            ⊥       0.0032                                                           //           0.0207                                             ______________________________________                                    

In Table II, the measurements were taken in planes normal and parallelto the fibers and the conductivity was measured through the composite. ⊥refers to the measurements made in a direction normal to the plane ofthe fibers and ∥ refers to the measurements made in a direction parallelto the plane of the fibers.

Because of the low temperature environment which high performance lasermirrors may be used in (e.g. outer space applications), testing wasperformed at 150 K and 300 K to demonstrate the composite strength andfracture toughness of the laser mirror under such adverse temperatureconditions. In all cases, the mirrors were able to withstand high loadswith increasing strain after initial fracture, and in all cases themirrors remained substantially intact at the conclusion of testing.Attention is directed to Table III for demonstration of the superiorstrength and toughness of the mirror composites of the presentinvention.

And as far as high temperature applications of the mirrors of thepresent invention, the only limitation would be the upper limits of thereflective layers which are applied. Currently, most protective coatingshave an upper limit of about 300° C. before they begin to release fromthe surface. However, because of the high temperature stability of thecomposites of the present invention, reflective layers with temperaturelimits greater than this could also be used, resulting in a coatedmirror with higher use temperature than currently used.

Presently, the two most popular materials for laser mirror use aremolybdenum substrates with intricate cooling passages and gold or silvercoated ULE glass. However, the composites of the present invention arefar superior to either of these commercially known materials. Themirrors of the present invention have a thermal conductivity sufficientfor high flux loads, a low thermal expansion, are low density andlightweight.

                  TABLE III                                                       ______________________________________                                        Three Point Flexural Strength of Unnotched                                    0/90 Cross Ply Reinforced Glass                                                                     Flatwise                                                          Test Temperature                                                                          Flexural Strength                                       Specimen Number                                                                           K             MPa      10.sup.3 psi                               ______________________________________                                        1           300           294      42.6                                       2           300           316      45.9                                       3           300           367      53.3                                       4           300           301      43.6                                        5*         300           259      37.5                                        6*         300           331      48.0                                                   Average       311      45.5                                       7           150           192      27.8                                       8           150           354      51.3                                       9           150           214      31.1                                       10          150           485      70.3                                                   Average       311      45.1                                       ______________________________________                                         *Specimens 5 and 6 were tested with machined surface in tension while all     others had the asfabricated surface in tension.                          

Such properties provide low distortion with increased flux, minimumcoolant system requirements, and superior optical component durability.Such properties can result in mirrors fabricated with a thermalexpansion only 3% that of molybdenum, with a lightweight density lessthan aluminum, and a cost only 10%, for example, of conventionalmolybdenum mirrors. Furthermore, the mirrors of the present inventioncan be made with a 90% reduction in distortion, 60% reduction in coolantsupply, and 90% reduction in weight over conventionally used molybdenummirrors.

It should also be noted that while this disclosure is specificallydirected to laser mirrors, it would be within the purview of one skilledin this art to use the mirrors of the present invention to reflect otherwavelengths of radiation such as optical light.

It is quite surprising that two brittle materials such as graphite andglass can produce in combination such a fracture tough, dimensionallystable mirror. For example, all glass mirrors have to be cooled slowlyto avoid cracking. However, with the present invention the mirrors havesuch a high degrees of fracture toughness that such slow cooling is notnecessary. Furthermore, because of the high thermal conductivity of suchmirrors, cooling takes place not only much more uniformly, but much morequickly as well. And the dimensional stability of such mirrors is suchthat they are particularly suitable for close tolerance work. Forexample, a two-inch wide, two-inch thick mirror made according to thepresent invention would change only 0.05 mil in width and only one milin thickness over a 100° C. change in temperature. And the high fracturetoughness allows machining to almost any design. The presence of thefibers inhibits microcrack formation during machining and during use.

Although this invention has been shown and described with respect to apreferred embodiment, it will be understood by those skilled in this artthat various changes in form and detail thereof may be made withoutdeparting from the spirit and scope of the claimed invention.

Having thus described a typical embodiment of our invention, that whichwe claim as new and desire to secure by Letters Patent of the UnitedStates is:
 1. A laser mirror comprising a graphite fiberglass matrixcomposite having a graphite fiber lay-up in the glass matrix producing asymmetrical and balanced composite, containing 40%-70% volume graphitefibers, having an outer laser reflecting layer, and having a graphitefiber orientation in the composite of 0°/90°, 0°/45°/90° or 0°/60°.
 2. Alaser mirror comprising a graphite fiber-glass matrix compositecontaining 40% to 70% by volume graphite fibers, having a laserradiation reflecting outer layer and having a graphite fiber lay-up inthe glass matrix producing balance and a central plane of symmetryacross the central plane of the composite.
 3. The mirror of claims 2 or1 containing about 60% by volume graphite fiber.
 4. The mirror of claims2 or 1 wherein the glass comprises borosilicate.
 5. The mirror of claims2 or 1 wherein the graphite fiber has a modulus of elasticity of atleast 531 GPa, a tensile strength of at least 1724 MPa and a density ofabout 1.96 gm/cm³.